Amniotic fluid contains a high concentration of prolactin which is produced and secreted by the decidua. In vitro models have suggested that bacterial products inhibit prolactin secretion by decidual cells. To further examine this potentially important regulatory mechanism in the whole animal, chronically instrumented pregnant rhesus monkeys were prepared. Experimental infection was induced by intraamniotic (n=5) or choriodecidual (n=4) inoculation of 103 - 106 Group B streptococci. Serial amniotic fluid samples were obtained prior to and following inoculation. Samples were heparinized and frozen at -20xC until assay for prolactin. Prolactin levels were determined by RIA using a specific anti-monkey prolactin serum developed in this laboratory. All animals went into labor between 12 and 72 h following inoculation. Three animals that delivered within 24 h were excluded from further analysis because insufficient samples were obtained prior to delivery. Data was obtained from 4 animals with intraamniotic inoculation and 2 animals with choriodecidual inoculation. The average prolactin level prior to inoculation (0 h) equaled 34.0 q 6.4 fg/ml. There was a 40% decrease in prolactin by 37 h post-infection (n=6). In 3 animals with later delivery, prolactin levels decreased by 71% between 61-72 h post-infection. Histopathology confirmed chorioamnionitis and deciduitis. Time post-infection (h) O 1-12 13-24 25-36 37-48 49-60 61-72 Prolactin (fg/ml) 34.0 33.9 22.5 22.0 20.3 13.1* 9.7* SEM 6.4 5.5 4.0 4.4 4.5 3.5 2.4 Number of animals 6 6 6 6 6 4 3 *Significantly different from 0 time at p<0.05 (Anova and Duncan's pairwise comparison) Intrauterine bacterial infection decreases amniotic fluid prolactin prior to preterm labor. These results are consistent with previous observations in vitro and they offer the first in vivo report of a potential relationship between infection and amniotic fluid prolactin.